This invention relates to implantable electrodes in general and more particularly an improved implantable electrode particularly useful as a stimulation electrode, as well as its application.
Stimulation or stimulating electrodes, for instance, for heart pacemakers generally include an insulated cable lead and an electrode head for transmitting the stimulation pulses. The electric stimulation of the heart when the propagation of the stimulation is interrupted presupposes the generation of a certain electric field strength at an excitable cell membrane. After the stimulation is triggered, the latter propagates automatically over the entire heart muscle and causes the heart to contract.
To trigger the stimulus an electronic pacemaker which consists of an implantable electronics part having a power supply unit and a stimulation circuit including the stimulating electrode and an indifferent electrode is used. During the stimulation pulse a small capacitor is partially discharged through the stimulation circuit within 0.5 to 2 msec. In the intervals between the pulses, the capacitor is recharged from the power supply unit, i.e., a battery. During the pulse, the field strength required for triggering of the stimulus exists in the excitable tissue in the vicinity of the stimulation electrode.
Heretofore it has been customery to use stimulation electrodes of platinum or Elgiloy. However, these electrodes have the disadvantage of causing the adjacent tissue to degenerate, since they surround themselves with a connecting or connective tissue layer about 0.5 to 1 mm thick which is not excitable. This connective tissue layer develops over a period of approximately 3 to 4 weeks. During this time, the stimulation threshold continuously increases, i.e., an increasingly larger current is required for triggering the stimulation process. Thus, the voltage required also increases. The excitable tissue, so to speak, moves away from the electrode, and more energy must therefore by supplied to generate the same field strength at the surface of the virtual electrode formed in this manner as at the stimulation electrode itself. If the head of the stimulation electrode consists, for instance, of an hemisphere with a radius of 1 mm and a connecting tissue layer about 1 mm thick develops around this stimulation electrode, then the stimulation threshold current increases fourfold. Since the voltage increases approximately at the same rate, the power required becomes about 16 times as large. This means that the requirements placed on the capacity and the voltage of the energy source depend to a considerable degree on the tissue growth at the stimulating electrode.
Although it is known to use as electrode materials for stimulation electrodes spectrally pure graphite and carbon, such electrodes nevertheless have not found acceptance in practice. For, at the surface of these electrodes, at thin capsule of connective tissue also develops, and in addition, these electrodes do not withstand the mechanical stresses in the heart. In fact, the wear and the danger of breakage are so great, that they are not suited in the long run for human implantation.